Aircraft flight envelope protection and recovery autopilot

ABSTRACT

Systems and aircraft are provided. An avionics system for an aircraft includes: a trajectory selection module configured to select a potential aircraft path relative to a current aircraft flight condition; a trajectory flight condition module configured to estimate a modeled flight condition of the aircraft along the potential aircraft path; a limit comparison module configured to determine whether the modeled flight condition violates aircraft limits; and a violation indicator module configured to generate an indication of impending violation.

TECHNICAL FIELD

The present disclosure generally relates to aircraft flight envelope protection systems, and more particularly relates to aircraft flight envelope protections systems that model potential aircraft trajectories and test the trajectories for aircraft limit violations.

BACKGROUND

Aircraft are designed to operate within certain operating speeds and loads on control surfaces of the aircraft. These operating limits are known as the flight envelope, outside of which there may be damage or loss of control of the aircraft. In order to protect against operating outside of the flight envelope, conventional aircraft utilize many disparate systems that evaluate individual aspects of the aircraft to determine whether the aircraft is operating outside of the flight envelope or is likely to collide with the ground on the present flight path. These conventional systems, however, have limitations that prevent full envelope protection.

Furthermore, these conventional systems are often disabled for landing based on whether the landing gear is down or by pilot command. Disabling the systems for landing, however, causes the aircraft to lose flight envelope protection during the landing.

Accordingly, it is desirable to provide systems and aircraft that provide greater flight envelope protection during flight and during landing phases. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

SUMMARY

Systems and aircraft are provided for flight envelope protection. In a first non-limiting embodiment, an avionics system includes, but is not limited to, a trajectory selection module configured to select a potential aircraft path relative to a current aircraft flight condition; a trajectory flight condition module configured to estimate a modeled flight condition of the aircraft along the potential aircraft path; a limit comparison module configured to determine whether the modeled flight condition violates aircraft limits; and a violation indicator module configured to generate an indication of impending violation.

In a second non-limiting embodiment, an aircraft includes, but is not limited to, a sensor system configured to provide aircraft flight condition data, an actuator system configured to manipulate control surfaces of the aircraft, and a control system. The control system includes: a trajectory selection module configured to select a potential aircraft path relative to a current aircraft flight condition; a trajectory flight condition module configured to estimate a modeled flight condition of the aircraft along the potential aircraft path; a limit comparison module configured to determine whether the modeled flight condition violates an aircraft limit; and a violation indicator module configured to generate an indication of impending violation.

BRIEF DESCRIPTION OF DRAWINGS

Advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a schematic diagram illustrating an aircraft having a control system, in accordance with various embodiments; and

FIG. 2 is a dataflow diagram illustrating the control system of the aircraft of FIG. 1, in accordance with various embodiments.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and is not intended to limit the application and uses. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. As used herein, the term module refers to any hardware, software, firmware, electronic control component, processing logic, and/or processor device, individually or in any combination, including without limitation: application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.

Embodiments of the present disclosure may be described herein in terms of functional and/or logical block components and various processing steps. It should be appreciated that such block components may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. For example, an embodiment of the present disclosure may employ various integrated circuit components, e.g., memory elements, digital signal processing elements, logic elements, look-up tables, or the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. In addition, those skilled in the art will appreciate that embodiments of the present disclosure may be practiced in conjunction with any number of systems, and that the systems described herein is merely exemplary embodiments of the present disclosure.

For the sake of brevity, conventional techniques related to signal processing, data transmission, signaling, control, and other functional aspects of the systems (and the individual operating components of the systems) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in an embodiment of the present disclosure.

Various embodiments disclosed herein describe systems that implement a Trajectory Prediction Algorithm (TPA) and a Recovery Autopilot. The TPA models various possible recovery trajectories and tests those trajectories against the aircraft limits and terrain clearance. The recovery trajectories represent flight paths that will potentially guide the aircraft away from impending aircraft limit violations or terrain conflicts. If a trajectory violates a limit, the trajectory will be ruled out and not used. When only one possible recovery is available and that recovery is approaching a limit, the TPA will trigger the Recovery Autopilot to initiate that recovery and thereby avoid the impending envelope exceedance or terrain conflict. Multiple trajectories are utilized to avoid false warnings. For example, if a right turn could be used to avoid terrain but the system does not model right turns, the system will trigger a recovery when the straight-ahead trajectory intersects the terrain. If the crew was safely planning the right turn, it would be a nuisance that the system activated unnecessarily when a safe route existed. The TPA models the recovery beginning from the current aircraft state using current aircraft performance capability. For example, the TPA uses energy modeling that is based on ambient temperature and engine failure status. The Recovery Autopilot takes control of the aircraft and executes the directed recovery once triggered.

Referring now to FIG. 1, an example of an aircraft 100 is illustrated in accordance with some embodiments. Aircraft 100 includes a control system 110, a sensor system 112, and an actuator system 114, among other systems. Although aircraft 100 is described in this description as an airplane, it should be appreciated that control system 110 may be utilized in other aircraft, land vehicles, water vehicles, space vehicles, or other machinery without departing from the scope of the present disclosure. For example, control system 110 may be utilized in submarines, helicopters, airships, spacecraft, or automobiles.

Control system 110 is an avionics system configured to operate aircraft 100 and to evaluate various trajectories 120 a-f, as will be described in further detail below. Sensor system 112 includes one or more sensing devices that sense observable conditions of the exterior environment, the interior environment of aircraft 100, or operational conditions and status of aircraft 100. For example, sensor system 112 may include accelerometers, gyroscopes, RADARs, LIDARs, global positioning systems, optical cameras, thermal cameras, ultrasonic sensors, and/or other sensors.

Actuator system 114 includes one or more actuator devices that control one or more vehicle features. For example, actuator system 114 may include actuators that manipulate control surfaces on aircraft 100, extend or retract landing gear of aircraft 100, an/or move other components of aircraft 100.

Referring now to FIG. 2, and with continued reference to FIG. 1, control system 110 is illustrated in accordance with some embodiments. Control system 110 includes at least one processor and a computer readable storage device or media. The processor may be any custom made or commercially available processor, a central processing unit (CPU), a graphics processing unit (GPU), an auxiliary processor among several processors associated with control system 110, a semiconductor based microprocessor (in the form of a microchip or chip set), a macroprocessor, any combination thereof, or generally any device for executing instructions. The computer readable storage device or media may include volatile and nonvolatile storage in read-only memory (ROM), random-access memory (RAM), and keep-alive memory (KAM), for example. The computer-readable storage device or media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by control system 110 in controlling aircraft 100.

The instructions may include one or more separate programs, each of which comprises an ordered listing of executable instructions for implementing logical functions. The instructions, when executed by the processor, receive and process signals from the sensor system, perform logic, calculations, methods and/or algorithms for automatically controlling the components of aircraft 100, and generate control signals for actuator system 114 to automatically control the components of aircraft 100 based on the logic, calculations, methods, and/or algorithms. Although only one control system 110 is shown in FIGS. 1-2, embodiments of aircraft 100 may include any number of control systems 110 that communicate over any suitable communication medium or a combination of communication mediums and that cooperate to process the sensor signals, perform logic, calculations, methods, and/or algorithms, and generate control signals to automatically control features of aircraft 100. In various embodiments, one or more instructions of control system, when executed by the processor, models possible recoveries of the aircraft and tests those recoveries for Mach limits, Calibrated Airspeed limits, Angle of Attack limits, and terrain conflicts.

In the example provided, control system 110 includes a flight management system 205, a potential path generation module 210, a trajectory selection module 215, a trajectory flight condition module 220, a terrain database 221, a climb ability database 223, an aircraft limit database 224, a limit comparison module 225, a violation indicator module 230, and a recovery autopilot module 235.

Flight Management System 205 (FMS 205) manages a flight plan, as will be appreciated by those with ordinary skill in the art. In the example provided, FMS 205 generates a potential landing indicator 305 when a flight plan/clearance for aircraft 100 indicates a potential landing. For example, when an airport is entered as a waypoint in FMS 205, then FMS 205 may generate the potential landing indicator 305 when aircraft 100 approaches the airport waypoint. It should be appreciated that other criteria and modules may be utilized to generate potential landing indicator 305. For example, other modules may generate potential landing indicator 305 when landing gear is extended, when a runway is within a threshold distance from aircraft 100, or when other conditions are met that suggest a flight crew may attempt to land aircraft 100.

Potential path generation module 210 is configured to generate a plurality of trajectories 310 from which trajectory selection module 215 selects a potential aircraft path relative to a current aircraft flight condition. Each of the potential aircraft paths corresponds to a potential recovery trajectory the aircraft may fly when other potential paths become undesirable.

In the example provided, potential path generation module 210 is configured to generate the plurality of trajectories to cover at least six different directions for a potential escape recovery, such as trajectories 120 a-f. For example, trajectories 310 may include a straight-ahead path, a straight climb path, a left climb path, a right climb path, a left descend path, and a right descend path. As will be appreciated by those with ordinary skill in the art, the dangerously low speed hazard is most significant when in a nose high condition and the overspeed hazard is most significant in the nose low condition, so nose high recoveries and nose low recoveries are modeled to provide full envelope protection. It should be appreciated that additional or alternative paths may be utilized without departing from the scope of the present disclosure.

In the example provided, potential path generation module 210 generates both a left bank trajectory and a right bank trajectory using a balance between bank angle and severity of nose high attitude, as will be appreciated by those of ordinary skill in the art. In a nose low situation, elimination of aircraft bank aids in the recovery, yet in the nose high situation, the addition of bank aids in recovery. This balance is based on what a pilot would do. For example, if the aircraft was only slightly nose high, no bank at all may be the most appropriate recovery. Most pilots will balance the amount of bank used with severity of the nose high recovery such that the termination for the recovery is smooth. The potential path generation module 210 balances the bank angle based on creating the smoothest possible recovery without conflict between nose low and nose high cases.

Potential path generation module 210 is further configured to generate a landing path of the plurality of trajectories in response to a potential landing indicator. By including the landing path, control system 110 may continue to operate as described below even during landing without disabling the trajectory evaluation. Control system 110 stays active all the way to the runway by using a “safe landing inhibit.” As the aircraft nears the approach end of the runway, the system will be inhibited from commanding a recovery for ground collision threat if a safe landing is indicated. In other words, while the landing path does not violate a limit, the landing path is available for the pilot to fly. This inhibit leverages the capabilities of conventional runway overrun protection systems to identify a safe approach to a runway. Unsafe approaches will not be inhibited and full protection is retained.

Trajectory selection module 215 is configured to select a potential aircraft path 315 of trajectories 310 for trajectory flight condition module to evaluate. In the example provided, trajectory selection module 215 evaluates each potential aircraft path of trajectories 310 in turn, and is configured to select a next consecutive trajectory of the plurality of trajectories as the potential aircraft path upon a completed evaluation of a previous potential path. Trajectory selection module 215 selects each potential path of trajectories 310 to fully evaluate each potential path aircraft 100 may take.

Trajectory flight condition module 220 is configured to estimate a modeled flight condition 317 of the aircraft along the potential aircraft path. The modeled flight condition may indicate the airspeed, pitch, roll, yaw, and other conditions that may be used to determine whether aircraft 100 violates the aircraft limits. In the example provided, trajectory flight condition module 220 includes a vertical velocity module 240, an energy state module 245, an airspeed prediction module 250, and a terrain conflict module 255. Trajectory flight condition module 220 receives sensor data 316 from sensor system 112.

Vertical velocity module 240 is configured to calculate a vertical velocity of the aircraft on the potential aircraft path. For example, vertical velocity module 240 may calculate the vertical velocity based on a vector velocity and a descent angle provided by sensor system 112.

Energy state module 245 is configured to calculate an energy state of the aircraft on the potential aircraft path. Energy modeling permits accurate prediction of Mach, Airspeed, and Angle of Attack along the potential aircraft path. Multiple trajectories executing in faster than real time can be taxing on the processor, so the energy modeling is performed using a simple, accurate, and fast algorithm.

Energy state module 245 is further configured to calculate the energy state based on a rate of climb of the aircraft at full power and on a rate of descent of the aircraft at idle power. Specifically, energy state module 245 utilizes interpolation between two parameters. The first parameter is the rate of climb at full power and the second is the rate of descent at idle power. These two parameters define the entire span of energy gain/loss of the aircraft. The parameters are computed using table lookup or simplified modeling based on current configuration and flight conditions.

Energy state module 245 is further configured to calculate the energy state based on a current power setting of the aircraft, a current power capability of the aircraft, a speedbrake position on the aircraft, landing gear and flap settings of the aircraft, and an engine health of the aircraft. For example, energy state module 245 may predict the future energy state of aircraft 100 by interpolating between the maximum climb rate and the idle power descent rate at specific temperatures or other conditions and accounting for the aircraft configuration. This ability to predict energy states permits accurate transition between nose high or nose low recovery and a steady climb final segment. By utilizing a maximum climb rate and idle descent rate based at least in part on engine failure status, control system 110 provides accurate predictions whether all engines are operating or if engine failure occurs. Since the transition between nose high recovery or nose low recovery and final segment climb is determined by energy state, control system 110 can accurately model a nose high recovery even while nose low. For example, if in level flight above the single-engine service ceiling and an engine fails while near an aircraft limit, control system 110 will predict and execute a nose high recovery even though the nose is level or nose low. This is because at level flight above the single engine service ceiling, the aircraft is energy deficient and should descend even to avoid terrain. In some embodiments, the system uses a constant energy plane and a constant altitude to distinguish between nose high unusual attitudes and nose low unusual attitude. Accordingly, control system 110 may accurately avoid terrain that is above the single engine service ceiling of the aircraft while the aircraft is conducting a single engine drift down maneuver, as will be appreciated by those of ordinary skill in the art.

Airspeed prediction module 250 is configured to estimate an airspeed of the aircraft on the potential aircraft path based on the vertical velocity and the energy state. For example, airspeed prediction module 250 may find the difference between climb capability and the vertical velocity, then use throttle position to calculate a change in airspeed.

In some embodiments, each trajectory is evaluated by looping the following algorithm:

Begin Loop Model Recovery Autopilot response Adjust aircraft state to reflect autopilot response Extend trajectory 1 time slice Compute vertical velocity based on model trajectory Compute energy gain/loss given current conditions/power/speedbrake) Adjust model energy based on vertical velocity and energy gain/loss Compute airspeed and Mach of new energy state Test new condition for limits violation Min Speed Limit Mach Limit Max Speed Limit Terrain Clearance Loop until aircraft is clear

Terrain database 221 stores terrain data 320 for use by terrain conflict module 255. For example, terrain database 221 may utilize conventional commercially available terrain data that indicates the height and location of terrain. Terrain conflict module 255 is configured to determine whether the potential aircraft path indicates a terrain conflict.

Climb ability database 223 stores climb ability data 325. In the example provided, climb ability data 325 is the climb rate at full power and the descent rate at idle power of aircraft 100 at specific conditions, such as at specific temperatures and altitude.

Aircraft limit database 224 stores aircraft limit data 330. As used herein, the term “aircraft limit” means flight condition limits such as Mach limits, maximum airspeed limits, minimum airspeed limits, angle of attack limits, and other similar limits of aircraft performance. As used herein, the term “aircraft limit” specifically excludes terrain conflicts.

In the example provided, predetermined passenger comfort limits are the aircraft limits. For example, the limits are sufficiently aggressive to be nuisance free yet still provide protection while preventing injury to unsecured passengers. The recovery relies on auto-pilot like maneuvers with roll onset rates constrained and limited g-loading to prevent large lateral and vertical accelerations in the cabin. Use of bank during the recovery aids in minimizing nuisances without imposing additional accelerations on the passengers.

In some embodiments, aircraft capability limits are the aircraft limits. For example, the aircraft capability limits permit greater accelerations and loading than are permitted by passenger comfort limits.

Limit comparison module 225 is configured to determine whether the modeled flight condition violates aircraft limits. For example, if an airspeed of aircraft 100 along the potential path is indicated as exceeding a maximum airspeed of the aircraft limits, then limit comparison module 225 will determine that the modeled flight condition violates aircraft limits.

Violation indicator module 230 is configured to generate an indication of impending violation 335 based on the limit comparison. In the example provided, violation indicator module 230 is further configured to generate the indication of impending violation based on the terrain conflict. In some embodiments, the indication of impending violation 335 may be conveyed to the flight crew by visual representation on a display in a flight deck of aircraft 100.

Recovery autopilot module 235 is configured to guide the aircraft along the selected trajectory in response to the selected trajectory being a last trajectory of the plurality of trajectories to lack a violation indication. For example, if five of six modeled trajectories have an indication of violation, recovery autopilot module 235 will command aircraft 100 to fly along the remaining modeled trajectory when that trajectory has an indication of impending violation. To command the flight, recovery autopilot module 235 may send control commands 340 to actuator system 114 to manipulate control surfaces of aircraft 100.

As will be appreciated by those of ordinary skill in the art, if the aircraft experiences a serious upset from windshear or other factors that place the aircraft in a near inverted case while at extreme bank angles, control system 110 is configured to determine which way to roll out for recovery by generating potential paths for each direction. For example, if in right bank but nearly inverted and still rolling right, control system 110 will evaluate a potential path that rolls through the inverted attitude to wings level. Control system 110 then commences a nose low recovery, arresting the roll and initiating roll in the shorted direction to commence the recovery. By commanding the selected trajectory, control system 110 ensures that the evaluated trajectory is the recovery flown by aircraft 100. For example, when the recovery trajectory predicts a roll through recovery, the recovery autopilot executes a roll-through recovery rather than a non-roll through recovery.

In the example provided, recovery autopilot module 235 is configured to guide the aircraft using the algorithm:

If Nose high (Losing Speed) or Insufficient Maneuver margin: Commanded g = 0.8 g Commanded Bank = +60/−60 (as directed by TPA) Until Rollout Point Commanded Power = Full Commanded Speedbrake = retract If Nose Low (Gaining Speed) Commanded g = 1.2 g (0.8 g if bank >90) Commanded Bank = 0 (roll through in TPA directed direction) Commanded Power = Idle Commanded Speedbrake = Hold Speed Constant (within desired margin) Commanded g = Speed on elevator for Vclimb Commanded Bank = −30, 0, +30 (as directed by TPA) Commanded Power = Full Commanded Speedbrake = retract

While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the disclosure in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the disclosure as set forth in the appended claims and the legal equivalents thereof. 

What is claimed is:
 1. An avionics system for an aircraft, the avionics system comprising: a trajectory selection module configured to select a potential aircraft path relative to a current aircraft flight condition; a trajectory flight condition module configured to estimate a modeled flight condition of the aircraft along the potential aircraft path; a limit comparison module configured to determine whether the modeled flight condition violates an aircraft limit; and a violation indicator module configured to generate an indication of impending violation.
 2. The avionics system of claim 1, wherein the trajectory flight condition module comprises: a vertical velocity module configured to calculate a vertical velocity of the aircraft on the potential aircraft path; an energy state module configured to calculate an energy state of the aircraft on the potential aircraft path; an airspeed prediction module configured to estimate an airspeed of the aircraft on the potential aircraft path based on the vertical velocity and the energy state.
 3. The avionics system of claim 2, wherein the energy state module is further configured to calculate the energy state based on a rate of climb of the aircraft at full power and on a rate of descent of the aircraft at idle power.
 4. The avionics system of claim 3, wherein the energy state module is further configured to calculate the energy state based on a current power setting of the aircraft, a current power capability of the aircraft, a speedbrake position on the aircraft, a landing gear setting, a flap setting, and an engine health of the aircraft.
 5. The avionics system of claim 1, further comprising a potential path generation module configured to generate a plurality of trajectories from which the trajectory selection module selects the potential aircraft path, wherein the trajectory selection module is configured to select a next consecutive trajectory of the plurality of trajectories as the potential aircraft path upon a completed evaluation of a previous potential path.
 6. The avionics system of claim 5, wherein the potential path generation module is further configured to generate the plurality of trajectories to cover at least six different directions for a potential escape recovery.
 7. The avionics system of claim 5, wherein the potential path generation module is further configured to generate a landing path of the plurality of trajectories in response to a potential landing indicator, wherein the limit comparison module is configured to determine whether the aircraft will violate the aircraft limit by following the landing path.
 8. The avionics system of claim 7, further comprising a recovery autopilot configured to guide the aircraft along the potential aircraft path in response to the potential aircraft path being a last trajectory of the plurality of trajectories to lack a violation indication.
 9. The avionics system of claim 1, wherein the limit comparison module is configured to determine whether the modeled flight condition violates a predetermined passenger comfort limit as the aircraft limit.
 10. The avionics system of claim 1, wherein the limit comparison module is configured to determine whether the modeled flight condition violates a capability limit as the aircraft limit.
 11. The avionics system of claim 1, further comprising a terrain conflict module configured to determine whether the potential aircraft path indicates a terrain conflict, and wherein the violation indication module is further configured to generate the indication of impending violation based on the terrain conflict.
 12. An aircraft comprising: a sensor system configured to provide aircraft flight condition data; an actuator system configured to manipulate control surfaces of the aircraft; and a control system comprising: a trajectory selection module configured to select a potential aircraft path relative to a current aircraft flight condition; a trajectory flight condition module configured to estimate a modeled flight condition of the aircraft along the potential aircraft path; a limit comparison module configured to determine whether the modeled flight condition violates an aircraft limit; and a violation indicator module configured to generate an indication of impending violation.
 13. The aircraft of claim 12, wherein the trajectory flight condition module comprises: a vertical velocity module configured to calculate a vertical velocity of the aircraft on the potential aircraft path; an energy state module configured to calculate an energy state of the aircraft on the potential aircraft path; an airspeed prediction module configured to estimate an airspeed of the aircraft on the potential aircraft path based on the vertical velocity and the energy state.
 14. The aircraft of claim 13, wherein the energy state module is further configured to calculate the energy state based on a rate of climb of the aircraft at full power and on a rate of descent of the aircraft at idle power.
 15. The aircraft of claim 14, wherein the energy state module is further configured to calculate the energy state based on a current power setting of the aircraft, a current power capability of the aircraft, a speedbrake position on the aircraft, a landing gear setting, a flap setting, and an engine health of the aircraft.
 16. The aircraft of claim 12, wherein the control system further comprises a potential path generation module configured to generate a plurality of trajectories from which the trajectory selection module selects the potential aircraft path, wherein the trajectory selection module is configured to select a next consecutive trajectory of the plurality of trajectories as the potential aircraft path upon a completed evaluation of a previous potential path.
 17. The aircraft of claim 16, wherein the potential path generation module is further configured to generate the plurality of trajectories to cover at least six different directions for a potential escape recovery.
 18. The aircraft of claim 16, wherein the potential path generation module is further configured to generate a landing path of the plurality of trajectories in response to a potential landing indicator, wherein the limit comparison module is configured to determine whether the aircraft will violate the aircraft limit by following the landing path.
 19. The aircraft of claim 18, wherein the control system further comprises a recovery autopilot configured to guide the aircraft along the potential aircraft path in response to the potential aircraft path being a last trajectory of the plurality of trajectories to lack a violation indication.
 20. The aircraft of claim 12, wherein the limit comparison module is configured to determine whether the modeled flight condition violates a predetermined passenger comfort limit as the aircraft limit. 